Field of the Invention
This invention relates to a bonded substrate for isolating semiconductor elements by dielectrics in which a semiconductor element region is surrounded by dielectrics, and also to a method of manufacturing the same, and more specifically to a dielectric isolation bonded substrate for isolating semiconductor elements having a high withstand voltage of more than 500 V, and a method of manufacturing the same.
Description of the Related Art
A bonded substrate is formed by adhering semiconductor substrates which are completely different from each other. It is a new element structure which cannot be obtained by the conventional methods. FIG. 3 shows an insulating film bonded substrate of a conventional type, in which first Si substrate 10 and second Si substrate 11 are adhered to each other by SiO.sub.2 12 interposed therebetween. A trench dielectric isolating section made of SiO.sub.2 15 and polysilicon 13 is formed on the insulating film bonded substrate. Since the elements are isolated by the dielectrics, an isolation capacitance, a problem to the conventional P-N isolation, is greatly reduced.
In a semiconductor device using the conventional isolating film bonded substrate, a high voltage is applied to first Si substrate 10 in which a semiconductor element is formed, and a ground potential is applied to second Si substrate 11. Insulating film SiO.sub.2 12 interposed between substrates 10 and 11 maintains a potential difference therebetween. To impart a sufficient withstand voltage to the semiconductor element, insulating film SiO.sub.2 12 must be thick. For example, to maintain a withstand voltage of 2000 V, the SiO.sub.2 film must be 5 to 10 .mu.m thick. Such a thick SiO.sub.2 film is difficult to form. Moreover, the bonded substrate for dielectric isolation using such an insulating film as SiO.sub.2 12 is electrically unstable, because an electric field may leak from one substrate into the other. Further, to remove a masking SiO.sub.2 layer which has been stained during the Si RIE (Reactive Ion Etching) process, NH.sub.4 F etches SiO.sub.2 12 sandwiched between the substrates, as indicated by the dotted line in FIG. 3. The etched portion is not buried even by the subsequent manufacturing steps and a space remains as hollow 14, which decreases the strength of the substrate.